The present invention relates to fuel control system for internal combustion engines, and in particular to a method and system for controlling the air-fuel ratio of mixture supplied to the engine in a closed loop operational mode during warm-up periods to thereby reduce the harmful components of the emission during such periods.
In conventional closed loop fuel control systems the air-fuel ratio of mixture supplied to the engine is corrected in response to a feedback signal which represents the deviation of the concentration of oxygen in the exhaust emissions detected by a zirconia dioxide oxygen sensor from a reference point which usually corresponds to the stoichiometric air-fuel ratio. The internal impedance of the oxygen however exhibits a considerably high impedance value when temperature within the exhaust system is low during warm-up periods. This impedance decreases as a function of temperature to a low or normally operating value when the engine has warmed up. Therefore, the signal provided by the gas sensor having a high internal impedance value cannot be used as a valid feedback signal and the conventional practice is to suspend the closed loop mode until the engine has warmed up, tending to produce a considerable amount of noxious emissions during warm-up periods.